In a 10- to 12-inch frying pan over medium heat, stir onion and 1 1/2 teaspoons garlic in oil until onion is limp and starting to brown, about 10 minutes. Stir in curry powder, cumin, cayenne, and 1/2 teaspoon salt.
Add potatoes and 1 cup water and bring to a boil over high heat. Reduce heat, cover, and simmer, stirring occasionally, until potatoes are tender when pierced, about 15 minutes; if they start to stick, add 1 to 2 tablespoons water to pan as needed. Stir in parsley and more salt to taste.
Meanwhile, in a bowl, mix yogurt, cucumber, lemon juice, and remaining 1/2 teaspoon garlic. Add salt to taste.
Toast pocket breads lightly in a toaster and gently pull open. Spoon potato mixture equally into halves; spoon yogurt sauce over potatoes.